If these transmission lines are built with any of the commonly available PCB materials, their impedance will not be constant over frequencies up to about 2 GHz due to the changing value or er. This is even true for Rogers 4350. > [Original Message] > From: <gianguida@xxxxxxxx> > To: <hassan@xxxxxxxx>; <si-list@xxxxxxxxxxxxx> > Date: 1/17/2007 10:44:21 AM > Subject: [SI-LIST] R: Re: Question on varying the coupling ratio on a differential pair > > Hassan, > I beleive that > trasmission line in TEM region, such as Microstrip, > have characteristic impedance costant over frequency > perhaps this is why people think in terms of characteristic impedance > Giancarlo > > > -----Messaggio originale----- > Da: si-list-bounce@xxxxxxxxxxxxx per conto di Hassan O. Ali > Inviato: mer 17/01/2007 17.28 > A: si-list@xxxxxxxxxxxxx > Oggetto: [SI-LIST] Re: Question on varying the coupling ratio on a differential pair > > > Just to add to the discussion: I don't remember seeing any high-speed electrical medium > that presents one fixed value of characteristic impedance for all signal frequencies > that matter! Why then are people so obsessed with "impedances" as opposed to maximum > return loss in a bandwidth of interest? > > Best regards. > > Hassan. > > > On Jan 17, "Loyer, Jeff" <jeff.loyer@xxxxxxxxx> wrote: > > > > Bill, > > I'd like some clarification. Are you saying that, even though you keep > > the impedance continuous, you will see reflections (for a purely > > differential signal) at points where the coupling coefficient changes? > > I can see a problem where a differential pair passes over a break in the > > return path, but not for a continuous return path.=20 > > > > Even then, I wonder - if you maintained the same impedance by some > > magic... Say you went from a pcb to a twisted pair and back to a > > twisted pair, all with the same differential impedance. Would that ever > > inherently cause a problem? =20 > > > > Regarding the original question, I think that it's usually impossible to > > ensure the same coupling coefficient from driver to receiver, and don't > > think it's necessary. I don't remember a design guide that specifies > > both odd and even impedances (or differential impedance plus coupling > > coefficient), though they may specify a "reference" stackup (but not for > > the purpose of achieving some magical coupling coefficient). > > > > Connectors (and other sources of return current discontinuities, such as > > vias, solder balls, or packages) represent challenges in maintaining > > return paths, but I don't know of a reason why the coupling coefficients > > would be maintained on either side of those discontinuities. The > > connectors (or vias, etc.) already represent such a tremendous change in > > impedance and/or coupling coefficient that trying to reduce common mode > > reflections by maintaining the coupling coefficient before and after > > seems pointless. =20 > > > > As always, perhaps I've missed something - I'm open to further > > enlightenment. > > > > Jeff Loyer > > > > -----Original Message----- > > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] > > On Behalf Of Bill Wurst > > Sent: Monday, January 15, 2007 5:23 PM > > To: si-list@xxxxxxxxxxxxx > > Subject: [SI-LIST] Re: Question on varying the coupling ratio on a > > differential pair > > > > Wayne, > > > > Even if the differential impedance is 100 Ohms from the source to the=20 > > final load and the signal is purely differential (no common-mode=20 > > component, originating from either the source or skew within the pair),=20 > > there will be reflections. The question is, how significant are they. > > > > To better understand what is happening, you might refer to Dr. Johnson's > > > > second book, High-Speed Signal Propagation - Advanced Black Magic, pp.=20 > > 417-8, where he describes "the differential U-turn." The current loop=20 > > created in the "U-turn zone" has a magnetic field associated with it=20 > > which can cause EMI, coupling into nearby conductors, and create an=20 > > inductive discontinuity. > > > > If your transition times are slow enough relative to the electrical size > > > > of the "U-turn zone", you won't be able to "see" a reflection. > > > > Regards, > > -Bill > > > > > > /************************************ > > / billw@xxxxxxxxxxx / > > / / > > / Advanced Electronic Concepts, LLC / > > / www.aec-lab.com / > > ************************************ > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > > cookeway wrote: > > > Hi all, > > >=20 > > > I have a question concerning differential pair coupling ratios that=20 > > > is causing some debate here in our design group. If a differential=20 > > > pair with an impedance of 100 ohms connects through a backplane=20 > > > connector and into a backplane such that the impedance is=20 > > > consistently 100 ohms, but the coupling ratio of the return currents=20 > > > on the adjacent planes versus the return current using the compliment=20 > > > differential signal changes, will there be a reflection on the signal? > > >=20 > > > For example, if you have a strongly coupled differential pair of 100=20 > > > ohms on your motherboard which goes through a connector to connect to=20 > > > a loosely coupled 100 ohm differential pair on the backplane, is=20 > > > there a reflection? My gut feeling is that the reflections caused by=20 > > > the different coupling ratios would cancel each other out causing no=20 > > > net reflection. Am I right in thinking this? > > >=20 > > > Thanks, > > >=20 > > > Wayne Cooke > > > Hardware Engineer, Zeugma Systems Inc. > > > Suite 250, 13571 Commerce Parkway, Richmond, BC, Canada, V6V 2R2=20 > > > Tel: (604) 247-2828=20 > > > Fax: (604) 247-3251 > > > wcooke@xxxxxxxxxxxxxxxxx=20 > > >=20 > > ------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > > > or to administer your membership from a web page, go to: > > <a href='http://www.freelists.org/webpage/si-list'>http://www.freelists.org/web page/si- > list</a> > > > > For help: > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > > List FAQ wiki page is located at: > > <a href='http://si-list.org/wiki/wiki.pl?Si-List_FAQ'>http://si- > list.org/wiki/wiki.pl?Si-List_FAQ</a> > > > > List technical documents are available at: > > <a href='http://www.si-list.org'>http://www.si-list.org</a> > > > > List archives are viewable at: =20 > > <a href='http://www.freelists.org/archives/si- > list'>http://www.freelists.org/archives/si-list</a> > > or at our remote archives: > > <a href='http://groups.yahoo.com/group/si- > list/messages'>http://groups.yahoo.com/group/si-list/messages</a> > > Old (prior to June 6, 2001) list archives are viewable at: > > <a > > href='http://www.qsl.net/wb6tpu'>http://www.qsl.net/wb6tpu</a> > > =20 > > ------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > > > or to administer your membership from a web page, go to: > > <a href='http://www.freelists.org/webpage/si-list'>http://www.freelists.org/web page/si- > list</a> > > > > For help: > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > > List FAQ wiki page is located at: > > <a href='http://si-list.org/wiki/wiki.pl?Si-List_FAQ'>http://si- > list.org/wiki/wiki.pl?Si-List_FAQ</a> > > > > List technical documents are available at: > > <a href='http://www.si-list.org'>http://www.si-list.org</a> > > > > List archives are viewable at: > > <a href='http://www.freelists.org/archives/si- > list'>http://www.freelists.org/archives/si-list</a> > > or at our remote archives: > > <a href='http://groups.yahoo.com/group/si- > list/messages'>http://groups.yahoo.com/group/si-list/messages</a> > > Old (prior to June 6, 2001) list archives are viewable at: > > <a > > href='http://www.qsl.net/wb6tpu'>http://www.qsl.net/wb6tpu</a> > > > > > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > http://www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > List archives are viewable at: > http://www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > http://www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > List archives are viewable at: > http://www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: http://www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu